Lenka Müller

Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting.

Selective electron beam melting (SEBM) was successfully used to fabricate novel cellular Ti-6Al-4V structures for orthopaedic applications. Micro computer tomography (microCT) analysis demonstrated the capability to fabricate three-dimensional structures with an interconnected porosity and pore sizes suitable for tissue ingrowth and vascularization. Mechanical properties, such as compressive strength and elastic modulus, of the tested structures were similar to those of human bone. Thus, stress-shielding effects after implantation might be avoided due to a reduced stiffness mismatch between implant and bone. A chemical surface modification using HCl and NaOH induced apatite formation during in vitro bioactivity tests in simulated body fluid under dynamic conditions. The modified bioactive surface is expected to enhance the fixation of the implant in the surrounding bone as well as to improve its long-term stability.

Osteoblast response to biomimetically altered titanium surfaces

Bioinert titanium (Ti) materials are generally encapsulated by fibrous tissue after implantation into the living body. To improve the bone-bonding ability of Ti implants, we activated commercially pure titanium (cpTi) by a simple chemical pre-treatment in HCl and NaOH. Subsequently, we exposed the treated samples to simulated body fluid (SBF) for 2 (TiCT) and 14 days (TiHCA), respectively, to mimic the early stages of bone bonding and to investigate the in vitro response of osteoblasts on thus altered biomimetic surfaces. Sample surfaces were characterized by scanning electron microscopy, energy-dispersive X-ray analysis, cross-sectional transmission electron microscopy analyses, Fourier transform infrared and Raman spectroscopy. It was shown that the efflorescence consisting of sodium titanate that is present on pre-treated cpTi surfaces transformed to calcium titanate after 2 days in SBF. After 14 days in SBF a homogeneous biomimetic apatite layer precipitated. Human osteoblasts (MG-63) revealed a well spread morphology on both functionalized Ti surfaces. On TiCT, the gene expression of the differentiation proteins alkaline phosphatase (ALP) and bone sialo protein was increased after 2 days. On both TiCT and TiHCA, the collagen I and ALP expression on the protein level was enhanced at 7 and 14 days. The TiCT and the TiHCA surfaces reveal the tendency to increase the differentiated cell function of MG-63 osteoblasts. Thus, chemical pre-treatment of titanium seems to be a promising method to generate osteoconductive surfaces.

The structure of titanate nanobelts used as seeds for the nucleation of hydroxyapatite at the surface of titanium implants.

The sequence of steps of a chemical treatment having as its goal the induce of nucleation and the growth of hydroxyl carbonated apatite (HCA) at the surface of titanium implants was studied by scanning and transmission electron microscopy in cross-section. In the first step, an acid etching forms a rough titanium hydride layer, which remains unchanged after subsequent treatments. In the second step, soaking in an NaOH solution induces the growth of nanobelt tangles of nanocrystallized, monoclinic sodium titanate. In the third step, soaking in simulated body fluid transforms sodium titanate into calcium titanate by ion-exchange in the monoclinic structure. HCA then grows and embodies the tangled structure. The interfaces between the different layers seem to be strong enough to prevent interfacial decohesion. Finally, the role of the titanate structure in the nucleation process of HCA is discussed.

Accelerated Biomimetic Deposition of Bonelike Apatite on Fibrous Cellulose Templates

Fibrous cellulose templates are attractive candidates for the use as tissue engineering scaffolds due to their biocompatibility and the adjustable porosity. Nevertheless, a direct bond between cellulose and bone is not formed under physiological conditions. A simulated body fluid solution with a high degree of supersaturation (5*SBF) was used to accelerate the biomimetic formation of bonelike apatite on cellulose templates. After generating calcium phosphate nuclei on the cellulose fibers in 5*SBF with high Mg2+and HCO3 - concentrations the cellulose templates were immersed in a modified 5*M-SBF which was optimized in respect to crystal growth kinetics by reduced Mg2+and HCO3- concentrations. After 48 hours a hydroxy carbonated apatite (HCA) layer with a thickness of 20 µm was obtained.

Precipitation of Carbonated Calcium Phosphate Powders from a Highly Supersaturated SBF Solution

Hydroxy carbonated apatite (HCA) powders were prepared by precipitation from a modified SBF solution (5x M-SBF). The ionic concentrations were 5 times higher than in human blood plasma with the exception of Mg2+ and HCO3 - concentrations that were reduced in order to accelerate crystal growth. Spheroaggregates of HCA platelets with molar (Ca+Mg)/P ratios ranging from 1.44 to 1.56 were obtained after precipitation at 50 °C. The crystallite size in c-direction was approximately 20 nm and depending on the precipitation time a CO3 2- content of 1.8 to 5.2 wt.-% was determined. Using this low temperature precipitation method, HCA powders with a high specific surface area of 83 m2/g and a composition and crystallite size close to those of the mineral phase of human bone were obtained.

Preparation of Bioactive Cellulose/Hydroxyapatite Composite Tapes

A composite material consisting of cellulose and HAp was prepared using coagulation of a native cellulose suspension. Composite tapes with a HAp content below 50 vol.% exhibit a gradient of filler particles across the cross-section of the sample due to gravity force that causes sedimentation of HAp, as long as the viscosity of the suspension is below a critical level during the coagulation process. According to gravimetric and solution analysis as well as SEM, the filler content influences the amount and uniformity of HCA precipitated in the surface of the tape. With increasing content of filler in the cellulose matrix, the apatite growth from SBF is promoted, due to a higher amount of HAp particles that serve as nucleation sites.

Fabrication of Hydroxyapatite Ceramics with Interconnected Macro Porosity

Calcium phosphate bioceramics with an interconnective pore structure were produced by foaming of hydroxyapatite and methyl phenyl poly(silsequioxane) melts in the temperature range between 250 °C and 310 °C. The cellular structure of the resulting porous bodies were controlled by foaming parameters and filler load. A porosity of up to 92 % was achieved by decreasing the HAfiller amount and increasing the foaming temperature. Subsequent pyrolysis in air at temperatures of 900 °C and 1100 °C resulted in macroporous foams composed of HA and HA/b-TCP, respectively. The porous bodies with tailorable structure and composition are of interest for bone tissue engineering scaffolds and orthopedic implants.

Inherent Luminescence of Annealed Biomimetic Apatites

Biomimetic apatite coatings are widely used in orthopaedic applications to provide bioinert material surfaces with bioactive behaviour by means of initiating bone growth at the implant surface. In this study we manufactured biomimetic calcium phosphate coatings consisting of a calcium deficient carbonated apatite by immersing activated titanium platelets into simulated body fluid (SBF). The development of the crystal phases was monitored by X-ray diffractometry (XRD) in addition to Fourier-transform infrared (FT-IR) spectroscopy. After annealing in air up to 600 °C luminescence of the biomimetically derived apatite was observed. The photo-induced emission spectra were recorded in the range from 400-750 nm at excitation wavelengths ranging 238 to 450 nm. A blue (437 nm) and a green (556 nm) emission were found between 200 to 600 °C visually appearing white. The results are discussed in terms of chemical and crystallographic changes in the biomimetic calcium phosphate layer during heat treatment.

The Structure and the Evolution of Titanate Nanobelts, Used as Seeds for the Nucleation of Hydroxyapatite at the Surface of Titanium Implants

The phase transformations due to a sequence of chemical treatments leading to the nucleation and biomimetic growth of hydroxyl carbonated apatite (HCA) at the surface of titanium implants were studied by scanning and transmission electron microscopy in cross-section. In the first step, an acid etching forms a rough titanium hydride layer which remains unchanged after subsequent treatments. In the second step, soaking in a NaOH solution induces the growth of nanobelt tangles of nanocrystallized, monoclinic sodium titanate. In the third step, soaking in a simulated body fluid transforms sodium titanate into calcium and phosphorus titanate, by ion exchange in the same monoclinic structure. Then HCA, of a hexagonal structure, grows and embodies the tangled structure showing a preferential direction growth along its “c”-axis, perpendicular to the substrate surface. The interfaces between the different layers seem to be strong enough to prevent interfacial decohesion. The role of the titanate phase in the nucleation of HCA is finally discussed.

Carbonate Substitution and Preferred Growth Orientation of Biomimetic Apatites

Simulated body fluid (SBF) solutions are widely used for in vitro bioactivity tests and to coat bioinert materials with biomimetic calcium phosphates. In this study SBF solutions with varying HCO3 - content were used to precipitate hydroxy carbonated apatite (HCA) on a bioactive titanium surface. XRD as well as cross-sectional TEM analyses revealed that the biomimetically derived crystallites show a preferred growth orientation in direction of their c-axis and perpendicular to the surface of the substrate. FTIR and Raman analyses revealed that, as long as the HCO3 - concentration in the testing solutions is below 20 mmol/l, only B-type HCA precipitates. Using SBF with a HCO3 - concentration equal to human blood plasma (27 mmol/l) leads to a AB-type substitution where, in accordance with bone mineral, CO3 2- substitutes PO4 3- as well as OH-.